Add `clang` to host compiler candidates in cross building.

* builds/unix/configure.raw: In cross building,
`clang` is tried if there is no `gcc`, nor `cc`.

builds/unix/configure.raw

@@ -46,6 +46,7 @@ if test ${cross_compiling} = yes; then
   AC_CHECK_PROG(CC_BUILD, ${build}-gcc, ${build}-gcc)
   test -z "${CC_BUILD}" && AC_CHECK_PROG(CC_BUILD, gcc, gcc)
   test -z "${CC_BUILD}" && AC_CHECK_PROG(CC_BUILD, cc, cc, , , /usr/ucb/cc)
+  test -z "${CC_BUILD}" && AC_CHECK_PROG(CC_BUILD, clang, clang)
   test -z "${CC_BUILD}" && AC_MSG_ERROR([cannot find native C compiler])
 
   AC_MSG_CHECKING([for suffix of native executables])